How to Tell if a Molly Fish is Male or Female?

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The molly is one of the guppies’ cousins, alongside swordtails platies, which is another way of saying that they are peaceful, easy to maintain, and color-diverse.

Mollies display a variety of colors and patterns, making them ideal for community tanks, especially since they’re so friendly and acceptant of other species.

However, learning the difference between male and female mollies can make a difference when looking to set up a stable aquarium community.

Male mollies will become aggressive towards one another in crowded tanks or when there are not enough females available.

Today’s article will look at the fundamental differences between male and female mollies to show how you can separate them easier.

Molly Male vs. Female – How to Tell the Gender?

There are, generally speaking, 5 areas where mollies differ visibly past a certain age. These include:

Body Size

Generally speaking, female mollies grow larger than males.

Depending on the species, they will reach maximum sizes of up to 5 inches, while males will remain in the 2.5-3.5-inch zone.

However, these values can vary at times. The males can get bigger and display bulgier bellies if they suffer from diseases that cause bloating.

So, you should first eliminate any disturbing factors like diseases that could influence your analysis.

Aside from being larger in general, female mollies also display bigger bellies thanks to the existence of their uterus.

The uterus forces the abdomen to take on a rounder shape, making the female appear rounder and more imposing.

By comparison, the males remain slimmer, shorter, and more compact.

Tail Fin Shape

The tail fin is pretty much similar in both genders if you ignore the difference in size.

This, however, is the natural result of a larger body overall, so we can’t really count it as a meaningful difference. The dorsal fin, however, is a different story.

Despite their reduced proportions, males have way larger and longer dorsal fins than females.

In most cases, the males’ dorsal fin is 3 to 4 times larger than that of females; there’s no way you will miss the difference. And the reason for that is evolutionary.

The dorsal fin makes the male molly appear larger and more intimidating towards other males.

This helps the male win its battles before they even start. The size of the dorsal fin is also the male’s business card that he will use to impress the female.

The female molly will consider the size of the male’s dorsal fin, among other things, when choosing who to mate with. Yes, size matters for mollies.

Anal Fin Shape

Mollies have different anal fins. The female mollies have wider and slightly longer anal fins that move freely in the water while they swim.

Males display a thorn-shaped fin that never opens wide, like in the case of females.

That’s because the male’s anal fin is actually a gonopodium. This organ is present in most fish species in males only.

They are copulatory organs that the males will use to deliver their sperm inside the female during mating.

The gonopodium may differ in size, depending on the male’s size and developmental phase, but it’s always slim and narrow with a pointy end.

The organ will retain the color of the male’s dorsal and caudal fins.


It’s true that mollies are easy-going, calm, and friendly fish, which qualifies them as great additions to community tanks.

It’s also true that male mollies can display quite significant levels of violence towards one another.

Mollies can tell the difference between members of their own species and the rest, causing males to display targeted violence towards other male mollies.

The main reason for that is generally reproductive competition.

Mollies mate every month, which means males are almost always in reproductive competition, especially when there aren’t enough females around.

This is why you should keep at least 3 females for every male, reducing the male’s sexually-infused aggressive behavior.

Male mollies will also fight over food, space, and even with no other reason than pure male-specific dominance.

You can mitigate the male’s behavior by decorating your tank with plants and decorations designed to break the line of sight between aggressors and victims.

On the other hand, females are more peaceful in nature and won’t seek too much conflict.

Except when they’re pregnant and get cranky as the delivery term approaches.

Gravid Spot

If you couldn’t tell the difference between your male and female mollies by the time, they got pregnant, now you can.

The gravid spot is a female-specific phenomenon consisting of a darker coloration visible in the female’s belly.

The gravid spot will appear in the lower portion of the abdomen, towards the cloaca, about halfway through the pregnancy (around 14 days in).

This phenomenon is typical in livebearers, and it is the most common indicator of the fish’s sex since only female mollies can get pregnant.

Expect the gravid spot to grow in size and intensity (become darker) towards the end of the pregnancy.

Some molly keepers claim that you can even distinguish the fry’s eyes in the same region when the female is ready to deliver.

When Can You Tell the Gender of Molly Fry?

Mollies will begin to display visible sex differences between 4 to 6 months.

However, these are general timeframes, not universal, because some mollies are known as late bloomers.

The term is common in livebearers, as well as fish in general. Some males will mature later than others, gaining their male-specific characteristics when they’re 8 months old or even older.

If you expect to differentiate mollies’ sex at birth, keep expecting because that’s not going to happen.

However, once you can tell which is which, you must separate them by sex. Keep 1 male for every 3-4 females for optimal population dynamics.

Can Molly Fish Change Gender?

This is the wrong question you want to post on the internet. Partly is because you will get opposing from people who are absolutely sure they are right.

Hence, the confusion which adds nothing to clear up your lack of knowledge.

In this sense, fish-related forums are probably your worst bet at getting reliable information.

To set things clear once and for all, mollies cannot change gender. Or, to put it like that, there’s no scientific evidence than they can.

All you have is anecdotical evidence from people claiming that they’ve seen their female mollies turning into males at some point.

Which says nothing, because how do you differentiate between a female turning into a male and an underdeveloped male who looked like one but wasn’t? You can’t.

So, take any such claims with a grain of salt and keep one thing in mind.

The current scientific consensus is that mollies cannot change gender since their gender is genetically defined, the same as humans.

Some mollies look like they change gender because of delayed male development, which can happen under certain circumstances.


Mollies don’t need much to live healthy and balanced lives.

The key aspect to take here is that learning how to tell mollies by gender can impact how you care for them.

Ensure a healthy male-to-female ratio, and your molly population will remain stable and thriving over the years.

And, oh yea, mollies can’t change gender. I’m out.

Author Image Fabian
I’m Fabian, aquarium fish breeder and founder of this website. I’ve been keeping fish, since I was a kid. On this blog, I share a lot of information about the aquarium hobby and various fish species that I like. Please leave a comment if you have any question.
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